Decoherent Histories and Realism

Rasmus Jaksland

Abstract This paper reconsiders what implications quantum decoherence has for Karen Barad’s agential realism. In contrast with the recent claim of Thomas Everth and Laura Gurney (2022), this paper argues that decoherence supports rather than defeats the holist, relational ontology of agential realism. Indeed, decoherence offers an explanation for how a quantum system can remain entangled and superposed in principle while it nevertheless in practice appears classical to a local observer. Decoherence shows why the appearance of classicality is not an objection to the ontology being in reality that of agential realism, in accordance with Barad’s repeated insistence that we should not mistake principle for practice. Whether users of agential realism in social theory should be encouraged by this is another matter that this paper does not take a stance on. As an ontology, however, agential realism is vindicated.

Slobodan Prvanović, Z. Marić

Alastair Wilson

Abstract This chapter sets out the version of quantum mechanics—diverging EQM—that constitutes the physics component of quantum modal realism. After a potted history of Everettian quantum mechanics, and an introduction to recent decoherence-based versions of the approach, I sketch a distinctive metaphysical understanding of Everettian quantum mechanics that involves diverging ‘parallel worlds’ instead of overlapping ‘splitting worlds’. I argue that the choice between divergence and overlap is underdetermined by the physics itself; I then adapt an argument of David Lewis’s that supports divergence over overlap, emphasizing the need to make room for objective probabilities in the Everettian picture. The chapter concludes by rejecting some unpersuasive motivations for overlap.

Claus Kiefer

Alastair Wilson

Abstract Quantum modal realism is a strange and unfamiliar doctrine. That is not by itself a reason to disbelieve it. Physical reality has continually turned out to be larger than we imagined possible; quantum modal realism expands reality one step further, beyond the actual world. In this short conclusion, I trace the history of human beliefs about the size of the cosmos, and compare the expansion of our horizons that results from quantum modal realism to previous revolutions in cosmology. I also observe that in some ways quantum modal realism is economical; while it may expand our conception of reality, it does not expand our conception of what is possible or of what is actual.

Claus Kiefer